Because you can read that book on multiple devices, with various screen sizes.
I have Kindle for my laptop and smartphone. I can read on one, stop, and pick up reading on the other seamlessly.
One page on my laptop is much more than one page on my smartphone, so the percentage is more relevant in these cases.
I started out with a Sony eReader. That displays pages but can also be read on a PC. The page numbers were different on the laptop than on the reader but page # was displayed on both. The Nook also displayed page numbers on both, even though the amount of information displayed on the PC was greater than on the Nook. So, that’s not the explanation...